Preparation and characterization of nanocrystalline Ce0.8Sm0.2O1.9 for low temperature solid oxide fuel cells based on composite electrolyte

Nanocrystalline Ce_0.8Sm_0.2O_1.9 (SDC) has been synthesized by a combined EDTA–citrate complexing sol–gel process for low temperature solid oxide fuel cells (SOFCs) based on composite electrolyte. A range of techniques including X-ray diffraction (XRD), and electron microscopy (SEM and TEM) have been employed to characterize the SDC and the composite electrolyte. The influence of pH values and citric acid-to-metal ions ratios (C/M) on lattice constant, crystallite size and conductivity has been investigated. Composite electrolyte consisting of SDC derived from different synthesis conditions and binary carbonates (Li₂CO₃–Na₂CO₃) has been prepared and conduction mechanism is discussed. Water was observed on both anode and cathode side during the fuel cell operation, indicating the composite electrolyte is co-ionic conductor possessing H⁺ and O²⁻ conduction. The variation of composite electrolyte conductivity and fuel cell power output with different synthesis conditions was in accordance with that of the SDC originated from different precursors, demonstrating O²⁻ conduction is predominant in the conduction process. A maximum power density of 817 mW cm⁻² at 600 °C and 605 mW cm⁻² at 500 °C was achieved for fuel cell based on composite electrolyte.     

Investigation of single SOEC with BSCF anode and SDC barrier layer

In this paper, Sm_0.2Ce_0.8O_1.9 (SDC) is used as a barrier interlayer between Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) anode and 8YSZ electrolyte to avoid solid state interaction of solid oxide electrolysis cells (SOEC) for high temperature application. The crystal structure and surface morphologies of BSCF and SDC powders were characterized, respectively. BSCF-SDC/YSZ/SDC-BSCF symmetric cells and BSCF-SDC/YSZ/Ni-YSZ single button cells were prepared and the related electrochemical performances were tested at 850 °C. The results showed that ASR data of BSCF-SDC/YSZ is 0.42 Ω cm² at 850 °C. The hydrogen production rate of the single SOEC using BSCF/SDC anode can be up to 177.4 mL cm⁻² h⁻¹, also the cell exhibits excellent stability, which indicates that it could be a potential candidate for the future application of SOEC technology.     

Study on BaCo0.7Fe0.2Nb0.1O3−δ—SDC composite cathodes for intermediate temperature solid oxide fuel cell

BaCo_0.7Fe_0.2Nb_0.1O_3−δ(BCFN)/Ce_0.8Sm_0.2O_1.9(SDC) composite material was prepared and characterized as cathode for intermediate temperature solid oxide fuel cells. The X-ray diffraction result proved that there was no obvious reaction between the BCFN and SDC after calcination at 1000 °C for 10 h. AC impedance spectra based on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ(LSGM) electrolyte measured at intermediate temperatures showed that a cathode with 30 wt% SDC exhibited the best electrochemical performance among the electrodes studied. The interfacial resistance value for BCFN/30SDC was as low as 0.0104, 0.017, 0.029, and 0.062 Ω cm² at 800, 750, 700 and 650 °C, respectively. The maximum power density of a single cell with BCFN/30SDC cathode, Ni_0.9Cu_0.1-SDC anode, and LSGM/SDC electrolyte was 209.7, 298.2, 407.1, 543.4 and 697.9 mW cm⁻² at 600, 650, 700, 750 and 800 °C.     

Electrochemical performance of (Ba0.5Sr0.5)0.9Sm0.1Co0.8Fe0.2O3−δ as an intermediate temperature solid oxide fuel cell cathode

This study presents the electrochemical performance of (Ba_0.5Sr_0.5)_0.9Sm_0.1Co_0.8Fe_0.2O_3−δ (BSSCF) as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFC). AC-impedance analyses were carried on an electrolyte supported BSSCF/Sm_0.2Ce_0.8O_1.9 (SDC)/Ag half-cell and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF)/SDC/Ag half-cell. In contrast to the BSCF cathode half-cell, the total resistance of the BSSCF cathode half-cell was lower, e.g., at 550 °C; the values for the BSSCF and BSCF were 1.54 and 2.33 Ω cm², respectively. The cell performance measurements were conducted on a Ni-SDC anode supported single cell using a SDC thin film as electrolyte, and BSSCF layer as cathode. The maximum power densities were 681 mW cm⁻² at 600 °C and 820 mW cm⁻² at 650 °C.     

Performance of fuel cells with proton-conducting ceria-based composite electrolyte and nickel-based electrodes

A ceria-based composite electrolyte with the composition of Ce_0.8Sm_0.2O_1.9 (SDC)–30 wt.% (2Li₂CO₃:1Na₂CO₃) is developed for intermediate temperature fuel cells (ITFCs). Two kinds of SDC powders are used to prepare the composite electrolytes, which are synthesized by oxalate coprecipitation process and glycine–nitrate process, respectively, and denoted as SDC(OCP) and SDC(GNP). Based on each composite electrolyte, two single cells with the electrolyte thickness of 0.3 and 0.5 mm are fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. With H₂ as fuel and air as oxidant, all the four cells exhibit excellent performances at 400–600 °C, which can be attributed to the highly ionic conducting electrolyte and the compatible electrodes. The cell performance is influenced by the SDC morphology and the electrolyte thickness. More interestingly, such composite electrolytes are found to be proton conductors at intermediate temperature range for the first time since almost all water is observed at the cathode side during fuel cell operation for all cases. The unusual transport property, excellent cell performance and potential low cost make this kind of composite material a good candidate electrolyte for future cost-effective ITFCs.     

A comparative study of Sm0.5Sr0.5MO3−δ (M = Co and Mn) as oxygen reduction electrodes for solid oxide fuel cells

Sm_0.5Sr_0.5MO_3−δ (M = Co and Mn) materials are synthesized, and their properties and performance as cathodes for solid oxide fuel cells (SOFCs) on Sm_0.2Ce_0.8O_1.9 (SDC) and Y_0.16Zr_0.92O_2.08 (YSZ) electrolytes are comparatively studied. The phase structure, thermal expansion behavior, oxygen mobility, oxygen vacancy concentration and electrical conductivity of the oxides are systematically investigated. Sm_0.5Sr_0.5CoO_3−δ (SSC) has a much larger oxygen vacancy concentration, electrical conductivity and TEC than Sm_0.5Sr_0.5MnO_3−δ (SSM). A powder reaction demonstrates that SSM is more chemically compatible with the YSZ electrolyte than SSC, while both are compatible with the SDC electrolyte. EIS results indicate that the performances of SSC and SSM electrodes depend on the electrolyte that they are deposited on. SSC is suitable for the SDC electrolyte, while SSM is preferred for the YSZ electrolyte. A peak power density as high as 690 mW cm⁻² at 600 °C is observed for a thin-film SDC electrolyte with SSC cathode, while a similar cell with YSZ electrolyte performs poorly. However, SSM performs well on YSZ electrolyte at an operation temperature of higher than 700 °C, and a fuel cell with SSM cathode and a thin-film YSZ electrolyte delivers a peak power density of ∼590 mW cm⁻² at 800 °C. The poor performances of SSM cathode on both YSZ and SDC electrolytes are obtained at a temperature of lower than 650 °C.     

Electrochemical performance of a solid oxide fuel cell based on Ce0.8Sm0.2O2−δ electrolyte synthesized by a polymer assisted combustion method

A polyvinyl alcohol assisted combustion synthesis method was used to prepare Ce_0.8Sm_0.2O_2−δ (SDC) powders for an intermediate temperature solid oxide fuel cell (IT-SOFC). The XRD results showed that this combustion synthesis route could yield phase-pure SDC powders at a relatively low calcination temperature. A thin SDC electrolyte film with thickness control was produced by a dry pressing method at a lower sintering temperature of 1250 °C. With Sm_0.5Sr_0.5Co₃-SDC as the composite cathode, a single cell based on this thin SDC electrolyte was tested from 550 to 650 °C. The maximum power density of 936 mW cm⁻² was achieved at 650 °C using humidified hydrogen as the fuel and stationary air as the oxidant.     

Effects of anode surface modification on the performance of low temperature SOFCs

An anode functional layer (AFL, ∼5 μm) for improving the cell performance was fabricated by the slurry spin coating method on the porous surface of an anode substrate. The effects of the AFL on the anode/electrolyte interfacial morphology and the Sm_0.2Ce_0.8O_1.9 (SDC) film deposition process were evaluated. And the electrochemical characteristics of the cells with and without the AFL were tested for comparison. With the AFL layer, the cell performance was greatly improved and the maximum power density was increased from 0.733 to 0.884 W cm⁻² at 600 °C and from 1.085 to 1.213 W cm⁻² at 650 °C. The systematical analysis indicated that the AFL could effectively reduce the anode polarization loss by increasing the three-phase boundary (TPB) length.     

High-performance PrBaCo2O5+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cell

PrBaCo₂O_5+δ-Ce_0.8Sm_0.2O_1.9 (PBCO-SDC) composite material are prepared and characterized as cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction result proves that there are no obvious reaction between the PBCO and SDC after calcination at 1100 °C for 3 h. AC impedance spectra based on SDC electrolyte measured at intermediate temperatures shows that the addition of SDC to PBCO improved remarkably the electrochemical performance of a PBCO cathode, and that a PBCO-30SDC cathode exhibits the best electrochemical performance in the PBCO-xSDC system. The total interfacial resistances R_p is the smallest when the content of SDC is 30 wt%, where the value is 0.035 Ω cm² at 750 °C, 0.072 Ω cm² at 700 °C, and 0.148 Ω cm² at 650 °C, much lower than the corresponding interfacial resistance for pure PBCO. The maximum power density of an anode-supported single cell with PBCO-30SDC cathode, Ni-SDC anode, and dense thin SDC/LSGM (La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ)/SDC tri-layer electrolyte are 364, 521 and 741 mW cm⁻² at 700, 750 and 800 °C, respectively.     

Preparation and characterization of new cobalt-free cathode Pr0.5Sr0.5Fe0.8Cu0.2O3−δ for IT-SOFC

A new cobalt-free perovskite oxide Pr_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (PSFC) has been synthesized and evaluated as cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The chemical compatibility of PSFC with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte has be proven by XRD, and its electrical conductivity reaches the maximum value of 264.1 S cm⁻¹ at 475 °C. Symmetrical cells with the configuration of PSFC/SDC/PSFC are used for the impedance study and the polarization resistance (Rp) of PSFC cathode is as low as 0.050 Ω cm² at 700 °C. Single cells, consisting of Ni–YSZ/YSZ/SDC/PSFC structure, are assembled and tested from 550 °C to 800 °C with wet hydrogen (∼3% H₂O) as fuel and static air as oxidant. A maximum power density of 1077 mW cm⁻² is obtained at 800 °C. All the results suggest that the cobalt-free perovskite oxide PSFC is a very promising cathode material for application in IT-SOFC.     

A high-performance no-chamber fuel cell operated on ethanol flame

A no-chamber solid-oxide fuel cell operated on a fuel-rich ethanol flame was reported. Heat produced from the combustion of ethanol thermally sustained the fuel cell at a temperature of 500–830 °C. Considerable amounts of hydrogen and carbon monoxide were also produced during the fuel-rich combustion which provided the direct fuels for the fuel cell. The location of the fuel cell with respect to the flame was found to have a significant effect on the fuel cell temperature and performance. The highest power density was achieved when the anode was exposed to the inner flame. By modifying the Ni + Sm_0.2Ce_0.8O_1.9 (SDC) anode with a thin Ru/SDC catalytic layer, the fuel cell envisaged not only an increase of the peak power density to ∼200 mW cm⁻² but also a significant improvement of the anodic coking resistance.     

Electrical properties of SDC–BCY composite electrolytes for intermediate temperature solid oxide fuel cell

The electrolyte material Ce_0.85Sm_0.15O_1.92 (SDC) powders are synthesized by glycine–nitrate processes and BaCe_0.83Y_0.17O_3−δ (BCY) powders are synthesized by sol–gel processes, respectively. Then SDC–BCY composite electrolytes are prepared by mixing SDC and BCY. The SDC and BCY powders are mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as SB95, SB90 and SB85, respectively. The electrical properties of SDC and SDC–BCY composites are investigated. The experimental results show that SDC–BCY composites exhibit the excellent conductivity and could significantly enhance the fuel cell performances. The behavior that SDC–BCY composites display hybrid proton and oxygen ion conduction is substantiated. Among these electrolytes, the maximum power density reaches as high as 159 mW cm⁻² for the fuel cell based on SB90 composite electrolyte at 600 °C.     

Electrochemical performance of a copper-impregnated Ni–Ce0.8Sm0.2O1.9 anode running on methane

Ni–Cu–Ce_0.8Sm_0.2O_1.9 anode-supported single cells were developed for the direct utilization of methane. An yttria-doped zirconia and Ce_0.8Sm_0.2O_1.9 bi-layer electrolyte and a La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ cathode layer were fabricated by slurry spin-coating. Cu was added to the anode by impregnation with a nitrate solution. The effects of Cu on the electrochemical performance of the anode were investigated in dry methane with respect to times of impregnation. Impregnation with Cu twice was determined to be optimal. Incorporating Cu into the anode improved electrochemical performance of the cells, reducing ohmic resistance and suppressing carbon deposition. At 700 °C, the single cell exhibited a maximum power density of 406 mW/cm² in dry methane. At a current density of 500 mA/cm², the cell maintained 98.6% of its initial voltage after operation for 900 min.     

Electrochemical performance of PrBaCo2O5+δ layered perovskite as an intermediate-temperature solid oxide fuel cell cathode

PrBaCo₂O_5+δ (PBCO) powder was prepared by a combined EDTA and citrate complexing method. The electrochemical performance of PBCO as a cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs) was evaluated. A porous layer of PBCO was deposited on a 42 μm thick electrolyte consisting of Ce_0.8Sm_0.2O_1.9 (SDC), prepared by a dry-pressing process. A fuel cell with a structure PBCO/SDC/Ni-SDC provides a maximum power density of 866, 583, 313 and 115 mW cm⁻² at 650, 600, 550 and 500 °C, respectively, using hydrogen as the fuel and stationary air as the oxidant. The total resistance of the cell was about 0.41, 0.51, 0.57 and 0.77 Ω cm², respectively. This encouraging data identifies PBCO as a potential cathode material for IT-SOFCs.     

Initialization of a methane-fueled single-chamber solid-oxide fuel cell with NiO + SDC anode and BSCF + SDC cathode

The initialization of an anode-supported single-chamber solid-oxide fuel cell, with NiO + Sm_0.2Ce_0.8O_1.9 anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ + Sm_0.2Ce_0.8O_1.9 cathode, was investigated. The initialization process had significant impact on the observed performance of the fuel cell. The in situ reduction of the anode by a methane–air mixture failed. Although pure methane did reduce the nickel oxide, it also resulted in severe carbon coking over the anode and serious distortion of the fuel cell. In situ initialization by hydrogen led to simultaneous reduction of both the anode and cathode; however, the cell still delivered a maximum power density of ∼350 mW cm⁻², attributed to the re-formation of the BSCF phase under the methane–air atmosphere at high temperatures. The ex situ reduction method appeared to be the most promising. The activated fuel cell showed a peak power density of ∼570 mW cm⁻² at a furnace temperature of 600 °C, with the main polarization resistance contributed from the electrolyte.     

Ceria-carbonate composite for low temperature solid oxide fuel cell: Sintering aid and composite effect

In this study, the effect of carbonate content on microstructure, relative density, ionic conductivity and fuel cell performance of Ce_0.8Sm_0.2O_1.9-(Li/Na)₂CO₃ (SDC-carbonate, abbr. SCC) composites is systematically investigated. With the addition of carbonate, the nano-particles of ceria are well preserved after heat-treatment. The relative densities of SCC pellets increase as the carbonate content increases or sintering temperature rises. Especially, the relative density of SCC2 sintered at 900 °C is higher than that of pure SDC sintered at 1350 °C. Both the AC conductivity and DC oxygen ionic conductivity are visibly improved compared with the single phase SDC electrolyte. Among the composites, SDC-20 wt% (Li/Na)₂CO₃ (SCC20) presents high dispersion, relative small particle size, and the dense microstructure. The optimized microstructure brings the best ionic conductivity and fuel cell performance. It is hoped that the results can contribute the understanding of the role of carbonate in the composite materials and highlight their prospective application.     

Comparative study on the performance of a SDC-based SOFC fueled by ammonia and hydrogen

A nickel-based anode-supported solid oxide fuel cell (SOFC) was assembled with a 10 μm thick Ce_0.8Sm_0.2O_2−δ (SDC) electrolyte and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode. The cell performance was investigated with hydrogen and ammonia gas evaporated from liquefied ammonia as fuel. Fueled by hydrogen the maximum power densities were 1872, 1357, and 748 mW cm⁻² at 650, 600, and 550 °C, respectively. While with ammonia as fuel, the cell showed the maximum power densities of 1190, 434, and 167 mW cm⁻², correspondingly. The power densities lower than that predicted, particularly at the lower operating temperatures for ammonia fuel cell, compared to hydrogen fuel cell, could be attributed to actual lower temperature than thermocouple display due to endothermic reaction of ammonia decomposition as well as the rather larger inlet ammonia flow rate. The results demonstrated that the ammonia was a right convenient liquid fuel for SOFCs as long as it was keeping the decomposition completion of ammonia in the cell or before entering the cell.     

Properties and performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 composite cathode

The properties and performance of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) + Sm_0.2Ce_0.8O_1.9 (SDC) (70:30 in weight ratio) composite cathode for intermediate-temperature solid-oxide fuel cells were investigated. Mechanical mixing of BSCF with SDC resulted in the adhesion of fine SDC particles to the surface of coarse BSCF grains. XRD, SEM-EDX and O₂-TPD results demonstrated that the phase reaction between BSCF and SDC was negligible, constricted only at the BSCF and SDC interface, and throughout the entire cathode with the formation of new (Ba,Sr,Sm,Ce)(Co,Fe)O_3−δ perovskite phase at a firing temperature of 900, 1000, and ≥ 1050 °C, respectively. The BSCF + SDC electrode sintered at 1000 °C showed an area specific resistance of ∼0.064 Ω cm² at 600 °C, which is a slight improvement over the BSCF (0.099 Ω cm²) owing to the enlarged cathode surface area contributed from the fine SDC particles. A peak power density of 1050 and ∼382 mW cm⁻² was reached at 600 and 500 °C, respectively, for a thin-film electrolyte cell with the BSCF + SDC cathode fired from 1000 °C.     

High performance Ni–Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells

A novel anode consisting of Ni and Sm₂O₃ with negligible oxygen-ion conductivity was developed for intermediate-temperature solid oxide fuel cells (SOFCs). Its triple phase boundary length is pretty small compared with the conventional Ni-samaria doped ceria (SDC) anode, of which SDC is one of the electrolytes having high oxygen-ion conductivity. Even so, single cells with Ni–Sm₂O₃ anodes generated peak power density of 542 mW cm⁻² at 600 °C, comparable to, if not higher than those with the Ni–SDC anodes when the same cathodes and electrolytes were applied. In addition, Ni–Sm₂O₃ exhibited lower interfacial polarization resistance than Ni–SDC. The high electrochemical performance, which might be related to the high catalytic activity of Sm₂O₃ and the unique microstructures of the Ni–Sm₂O₃, suggests a viable alternative to the conventional anodes for SOFCs.     

Effect of SDC-impregnated LSM cathodes on the performance of anode-supported YSZ films for SOFCs

Sm_0.2Ce_0.8O_1.9 (SDC)-impregnated La_0.7Sr_0.3MnO₃ (LSM) composite cathodes were fabricated on anode-supported yttria-stabilized zirconia (YSZ) thin films. Electrochemical performances of the solid oxide fuel cells (SOFCs) were investigated in the present study. Four single cells, i.e., Cell-1, Cell-2, Cell-3 and Cell-4 were obtained after the fabrication of four different cathodes, i.e., pure LSM and SDC/LSM composites in the weight ratios of 25/75, 36/64 and 42/58, respectively. Impedance spectra under open-circuit conditions showed that the cathode performance was gradually improved with the increasing SDC loading. Similarly, the maximum power densities (MPD) of the four cells were increased with the SDC amount below 700 °C. Whereas, the cell performance of Cell-4 was lower than that of Cell-3 at 800 °C, arising from the increased concentration polarization at high current densities. This was caused by the lowered porosity with the impregnation cycle. This disadvantage could be suppressed by lowering the operating temperature or by increasing the oxygen concentration at the cathode side. The ratio of electrode polarization loss in the total voltage drop versus current density showed that the cell performance was primarily determined by the electrode polarization. The contribution of the ohmic resistance was increased when the operating temperature was lowered. When a 100 ml min⁻¹ oxygen flow was introduced to the cathode side, Cell-3 produced MPDs of 1905, 1587 and 1179 mW cm⁻² at 800, 750 and 700 °C, respectively. The high cell outputs demonstrated the merits of the novel and effective SDC-impregnated LSM cathodes.